Tunable infrared (IR) lasers and heterodyne receivers have been used in the 3 to 5 and 8 to 12 .mu.m atmospheric windows to monitor ozone and other trace constituents. Standard direct detection techniques have limitations due to detector noise or background thermal noise, which make measurements with high spectral resolution difficult. (See "Atmospheric Monitoring Using Heterodyne Detection Techniques", R. T. Menzies, Optical Engineering, V17N1, Jan/Feb 1978, p. 44.)
Conventional IR heterodyne spectroscopy combines radiation from a coherent local oscillator (LO), such as a CO.sub.2 laser, with source radiation on a nonlinear detector to produce an electrical intermediate-frequency (IF) output signal lying in the microwave spectrum.
The best detector for this application has heretofore been the mercury cadmium telluride (HgCdTe) detector. (See "Infrared Heterodyne Spectroscopy", Mumma et al., Optical Engineering, V21N2, Mar/Apr 1982, p. 13.
The maximum electrical bandwidth B.sub.IF of the best HgCdTe detectors is about 3 GHz, which prevents spectral monitoring at frequencies more than B.sub.IF away from the LO frequency. Thus when the LO is a CO.sub.2 laser, most of the infrared radiation between 9 and 11 microns can not be detected because the adjacent CO.sub.2 emission lines are separated by much more than 3 GHz. Typically the CO.sub.2 line separation is at least 30 GHz (1 cm.sup.-1).